When hardness of steel pipe is below 500HB, which is proportional to the tensile strength and hardness. Hardness with tempering temperature decreases, but in the state as well as the quenching and tempering below 300 ℃, the hardness and tensile strength of the relationship is difficult established. When the tempering temperature at around 300 ℃, kg / m ㎡ with HRC is a correlation between that high hardness, tensile strength is high; low hardness, tensile strength is low.